APAC 2020 – Preview Paper: The Fremantle Underwater Rock Mound

As we all have to wait a few more months for our inaugural APAC 2020 Conference in Fremantle (30 Nov to 4 Dec), we thought we’d provide sneak previews of just some of the excellent papers being presented there…

The Fremantle Underwater Rock Mound –

Preventing Ship Impact with the Fremantle Rail Bridge

The Fremantle Rail Bridge (FRB) is located at the eastern end of the Fremantle Inner Harbour, which is Western Australia’s largest and busiest container and general cargo port.  The bridge, which dates from the early 1960s, provides regular access across the Swan River between Fremantle and North Fremantle for passenger and freight trains.

In 2011 the 65m BP Bunker vessel, Parmelia, was caught in the flood tide when leaving Berth 12A and came into contact with the FRB causing damage to the mast supporting the overhead powerlines for the rail system.  In 2014, a second incident occurred, during a storm event with strong winds, when the 150m long general cargo carrier, AAL Fremantle, partially broke away from its mooring at Berth 12, impacting the FRB and causing localised damage to the structure.

Following these two events, Fremantle Ports (FP), in conjunction with the Public Transport Authority (PTA) who own the FRB, decided that a protection structure was required to limit the risk of future vessel impacts.  A range of barrier options were considered to prevent a further collision, with the selected option being a rock structure.  (FP also made operational changes including installation of shoretension units to prevent vessels from breaking mooring lines and coming off the berths during storm events.)

The harbour is at the mouth of the Swan-Canning estuary and therefore experiences strong tidal currents as well currents created by other meteorological processes (eg river flood events, meteo-tsunamis, storm surges etc).  In addition to these currents, tug wash can create strong localised currents at the site.  Any rock structure would have to be able to deal with these conditions as well as shipping vessels up to 265m length and 80,000t displacement, and Vessel Impact energy of 3,000kNm.

The capacity of the structure to stop a ship before it reaches the FRB was assessed using results of physical model testing as well as various calculations of a vessel displacing rocks as it penetrates and displaces the structure.  A crest height of +0.8mLWMF was determined to provide an appropriate level of protection to enable grounding of the expected range of vessels.

Structural Design

The design currents and resulting armour rock grading was assessed using the PIANC Guidelines (MarCom WG180) for Protecting Berthing Structures from Scour Caused By Ships. The resulting design cross section comprises limestone armour rock over limestone core material.

Physical modelling of the structure was completed at the Manly Hydraulic Laboratory in Sydney to further refine and optimise the rock design.  To assess the impact of the structure on the hydrodynamics and sediment movement at the site a detailed numerical model was set up using Delft3D.  The results of this modelling confirmed that the potential impacts from the installation of the structure could be managed.  Construction then followed.

The new underwater rock mound is located at the eastern end of the Inner Harbour in a water depth of around 6 to 10m.  The mound provides a robust design which has greatly reduced the risk of large commercial ship impacts with the FRB.

Construction images provided by Fremantle Ports

Peter Doust, & Liam DeLucia, M P Rogers & Associates and Glen Pikor, Fremantle Ports, Perth,